In order the order of how many units each one sold here are my top 3. the sales numbers come from Wikipedia
1. Tiger Game.com (yes I really own one and I still do)! Sales fewer than 300,000
2. Neo Geo Pocket Color (I love this thing so hard) Sales 2 million
3. Gameboy Micro (it might be even better than the SP)Sales 2.42 million
I am hoping to get a Tiger Telematics Gizmondo and a Gameboy Light sometime in the future. I know the Gizmondo sold less than 25,000 units, but I not sure how many the Light sold either way those two will (hopefully) be the rarest I ever get.

“A thing may be too sad to be believed or too wicked to be believed or too good to be believed; but it cannot be too absurd to be believed in this planet of frogs and elephants, of crocodiles and cuttle-fish.”
― G.K. Chesterton

Sega Saturn....Had it for a few months or so and traded it in for a the playstation during it's launch period. Best move i ever did.
Plus i still have my VIrtual Boy, but it's broken. I also i had a Mint CIB US copy of the incredibly rare Jack. Bros. Sold it for about $90

Which was quite fun,,,, with the exception of the controller and the stupid plastic covers you would put over the # buttons and each game had their on plastic game controller cover... and you either lost them or they got torn.. which means it was hard to play any game with out them